Field of the Invention
This invention relates to a membrane bioreactor (“MBR”) system to treat wastewater. More particularly, the present invention relates to a MBR system that employs a repetitive back and forth motion (hereafter called the “reciprocating motion” or “reciprocation”) of a submerged membrane to increase filtration and nutrients removal efficiencies instead of membrane air scouring which is commonly utilized in submerged MBR.
Description of the Background Art
The background art contains several examples of MBR systems. These systems utilize biological treatment processes (e.g. activated sludge processes) to remove contaminants from wastewater. Several modified activated sludge processes can be used alone or in series for improved removal of nutrients in the MBR. Known MBR systems also use low pressure microfiltration (MF) or ultrafiltration (UF) membranes as a physical barrier for a complete solid-liquid separation. The UF or MF membranes can be submerged in a bioreactor or external to the bioreactor. Submerged membranes are typically installed in an aerobic bioreactor or a separate membrane tank. Membrane air scouring is of utmost importance in submerged MBR operation to prevent severe and rapid membrane fouling. By way of these known techniques, MBR systems can achieve secondary and tertiary wastewater treatment.
One advantage of known MBR systems is the direct production of tertiary quality effluent with the treatment of domestic or industrial wastewater. Another reason for the growing interest in MBR technology is its smaller footprint compared to conventional treatment processes. For example, using conventional MBR systems, a treatment plant could potentially double its capacity without increasing its overall footprint. MBR technology is not only limited to domestic wastewater, but it can also be applied to treat industrial wastewater for reuse.
An example of an MBR system is disclosed in U.S. Pat. No. 4,867,883 to Daigger. This reference discloses a high-rate biological waste water treatment process for removing organic matter, phosphorus and nitrogen nutrients from municipal waste water. A further MBR system is disclosed is U.S. Pat. No. 8,287,733 to Nick et al. It discloses a system utilizing first and second anoxic basins and first and second aerobic basins. Also disclosed is the use of a membrane chamber for housing a plurality of membrane tanks.
One common drawback of known MBR systems is membrane fouling. This occurs when soluble and particulate materials accumulate on the membrane surface. When such fouling occurs, there is either a marked decline in permeate passing through the membrane or an increase in the transmembrane pressure. In either event, the result is a dramatic reduction in system performance. Membrane fouling is especially problematic in MBR systems given that they generally operate with higher mixed liquor suspended solids (“MLSS”).
One solution to membrane fouling is air scouring. Vigorous air scouring allows for stable flux operation without rapid and permanent fouling and especially cake layer buildup. Given the higher MLSS concentrations at which MBR systems operate, frequent maintenance cleanings and out of tank cleanings are also important to maintain membrane performance in terms of fouling and permeability. Air scouring is not optimal as it is energy intensive. In MBR systems energy consumption is considerably higher than conventional activated sludge systems due to the additional air scouring for the membrane.
Thus, there exists a need in the art for improved MBR systems that eliminate or reduce membrane fouling and that do not rely upon air scouring. The present invention is aimed at fulfilling these and other needs in the art.